We wish to understand how mammalian cells react with mutagenic and carcinogenic agents. What is the response of the cell to the reaction products, how does mutation occur, what is the relationship between the processes of mutagenesis and carcinogenesis? The initial events in mutagenesis and carcinogenesis appear to result from DNA replication in the presence of lesions not removed by excision repair. We therefore want to understand the mechanisms of excision repair and of DNA replication in damaged cells and theri interactions. In the next year we propose to: 1) Determine the biochemical properties of the exonuclease involved in small patch apurinic repair in human lymphoblastoid cells. We have developed a system using polymerase and nucleases from human cells which along with ligase repairs MMS-induced damage. We intend to investigate the 5'-3' exonucleases found in such cells to understand the difference between base excision, small patch and nucleotide excision, large patch repair. 2) Determine the repair activity for two excision repair pathways of normal rodent liver cells as well as normal neural tissue to study the relationship between DNA repair and replication. 3) Determine the extent of single strandedness of DNA in cells damaged by carcinogen treatment and measure the size of the replicons in damaged cells to decide whether there is really a spearate process of post replication repair distinct from replication. 4) Attempt the hybridization of cells proficient in base excision repair to human xeroderma cells to determine whether the control mechanisms of the different cell types will complement one another and activate repair activity. We hope to investigate the factors which determine the level of repair activity in different tissues.